Uniform ionization means for mhd generators

ABSTRACT

Temperature limitations imposed by available materials require provision of other than purely thermal means for ionizing gas used in hot-gas magnetohydrodynamic (MHD) generators. Efficiency of such generators is markedly reduced by non-uniform ionization. Prior art ionization means, even when adequate, require use of appreciable fraction of length of expensive MHD channel to achieve high and uniform conductivity of gas plasma. Hot, cesiumseeded partially ionized gas passes through large-diameter conduit from source to nozzle feeding MHD channel. Magnetic field in pre-channel nozzle and conduit portions causes current flow in gas between short-circuited electrode pairs or pairs are fed repeated short high-voltage pulses continued by longer lower voltage pulses. Such current flow increases ionization everywhere in gas to desirable high and uniform level. Alternatively, gas not so seeded is ionized by irradiation from nuclear heat source; or gas not ionized is preionized by discharges between first pairs of electrodes, and then fully and uniformly ionized by discharges between later pairs of electrodes.

United States Patent 1 Zauderer 51 May 29, 1973 [54] UNIFORM IONIZATIONMEANS FOR MHD GENERATORS [75] Inventor: Bert Zauderer, Bala Cynwyd, Pa.

[73] Assignee: General Electric Company, New

York, NY.

[22] Filed: Sept. 27, 1971 [21] Appl. No.: 183,792

Primary Exa minerD. F. Duggan I Attorney-Allen E. Amgott, William G.Becker and Henry W. Kaufmann [57] ABSTRACT Temperature limitationsimposed by available materials require provision of other than purelythermal means for ionizing gas used in hot-gas magnetohydrodynamic (MHD)generators. Efficiency of such generators is markedly reduced bynon-uniform ionization. Prior art ionization means, even when adequate,require use of appreciable fraction of length of expensive MHD channelto achieve high and uniform conductivity of gas plasma. Hot,cesiumseeded partially ionized gas passes through largediameter conduitfrom source to nozzle feeding MHD channel. Magnetic field in pre-channelnozzle and conduit portions causes current flow in gas betweenshort-circuited electrode pairs or pairs are fed repeated shorthigh-voltage pulses continued by longer lower voltage pulses. Suchcurrent flow increases ionization everywhere in gas to desirable highand uniform level. Alternatively, gas not so seeded is ionized byirradiation from nuclear heat source; or gas not ionized is preionizedby discharges between first pairs of electrodes, and then fully anduniformly ionized by discharges between later pairs of electrodes.

5 Claims, 7 Drawing Figures 1 UNIFORM IONIZATION MEANS FOR MIIDGENERATORS The Invention herein described was made in the course of orunder a contract or subcontract thereunder, (or grant) with theDepartment of the Navy.

CROSS-REFERENCE TO RELATED APPLICATION TI-IERMIONIC CATHODES FOR MHDGENERA- TOR, by Bert Zauderer, Ser. No. 178,881, filed Sept. 9, 1971,assigned to the assignee of the present application.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionpertains to magnetohydrodynamic generators employing flowing ionizedgas.

2. Description of the Prior Art The ideal magnetohydrodynamic (hereafterMI-ID) generator would employ gas sufficiently hot to be a highly anduniformly ionized plasma; but this is not feasible for a continuouslyoperating device because the temperatures required would exceed themelting points of any materials available for its construction.Therefore it has been customary to seed the gas with cesium to promoteionization, and to operate at gas temperatures, e.g. 2000 K, which canbe borne by refractory metals and ceramics. The degree of ionizationproduced by such temperatures even in seeded gas is insufficient toproduce the high plasma conductivity required to produce high efficiencyin the generator. Therefore it has been known to provide opposedelectrodes, short-circuited in pairs, in a region of magnetic field, sothat the interaction between the charges which exist and the magneticfield will cause the flow of currents which will increase the existingionization. It is also known, where the original ionization isinadequate, to apply high-voltage pulses between pairs of electrodesinstead of short-circuiting them, in order that the intermittentbreakdowns which occur may furnish further ionization.

These methods have a number of disadvantages, as they have been applied.Since the basic process of converting thermal energy into electricalenergy involves heating the gas in a chamber to increase its pressureand then expanding it through a nozzle to convert the pressure energyinto kinetic, the resulting adiabatic temperature reduction militatesagainst ionization. Furthermore, although it has recently beenrecognized that high uniformity of ionization is necessary to highefficiency, the prior art methods of ionization have not directlyproduced uniform ionization across the cross section of a gas; to theextent that this has been produced it has occurred during the passage ofthe initially nonuniformly ionized gas through an appreciable length ofgenerator channel before it achieves a uniformity whose benefits areenjoyed only during passage through the downstream remainder of thechannel. Since MIID channel is expensive because of the particularlystringent requirements it must meet, this is wasteful. However, evenworse is the fact that the upstream part of the channel operatesinefficiently because of nonuniform ionization, and the losses therewill keep the total efficiency low however high the efficiency achievedat the downstream part.

SUMMARY OF THE INVENTION I provide ionization means which are at leastinitially operative upon the heated (and, preferably, cesiumseeded) gasat substantially stagnation conditionsthat it, at approximately thetemperature which exists in the hot gas source. This I achieve byleading the hot gas from the source in a thermally insulated duct ofsuch large crosssection that the gas velocity is not more than 0.] Mach;this minimizes the temperature drop produced by expansion of the gas toproduce the indicated velocity, and so preserves ionization at the levelcorresponding to approximately the stagnation temperature. I thenprovide means to increase the ionization of the gas during exparisionthrough a nozzle. Such means are applied in a relatively short nozzleleading into the MHD generator channel proper, and may extend into theinitial part of the channel itself. The virtue of initiating theionization enhancement in the nozzle is that the process operates uponthe gas prior to large adiabatic cooling. If the gas flow is subsonicthe nozzle entrance will have a continuously changing section, but forsupersonic flow a sharp corner may be used at the nozzle throat.

In a noble gas at a temperature of 2,000 K and a few atmospherespressure, seeded with cesium in a concentration of a fraction of apercent, the electron concentration is of the order of 10 per cubiccentimeter, which would be reduced by recombination if the gas werecooled by expansion and/or flow through a substantial length ofheat-lossy duct, which my invention largely avoids as I have described.A first means of enhancing this ionization which I teach is to provide amagnetic field (which may be a fringe of the field applied to the MI-IDchannel proper) transverse to the direction of flow through the nozzle;and on axes orthogonal to this magnetic field and to the flow directionto provide opposed pairs of electrodes, the cathodes beingthermionically emissive, segmented at least with respect to the flowdirection, or axis, and preferably also subdivided at right angles tothe axis. In any event, the electrodes are pairedthat is, for eachcathode there is a single opposed anode; and the electrodes of each pairmay be connected to each other, but individual pairs are insulated fromeach other. If the initial ionization is sufiiciently high and the gasvelocity and magnetic field are of sufficient magnitude, the currentflowing between electrodes in each pair will cause increase inionization, and will increase the average energy of the electrons sothat the ionization will not only be enhanced, but become relativelymore stable because the probability of capture of a high energy electronis low. Since the current flow tends to decelerate the gas flow, by anamount also proportional to the magnetic field intensity, it may be thatin a given design the magnitude of current flow required to produce anacceptable electron density (e.g. 5 X 10 per cubic centimeter or more)will be so great as to produce excessive decelera tion. In such case,the electrodes of each pair may be disconnected from each other andconnected to an individual source of high-potential pulses which willproduce large ionization of the gas lying between the electrodes andcurrent will flow from the source through the gas thus highly ionized.But since the gas is flowing, the highly ionized channel will be sweptaway from the electrodes, breaking the highly ionized path, andrequiring another high-potential discharge to start another pulse ofcurrent. This will have the undesirable effect of producing movingstriations of highly ionized gas lying transverse to the flow direction,with intervening portions of gas relatively little ionized. Thisnonuniformity is highly undesirable. Therefore I modify the highpotential source so that, after the ionized path has been established bythe high-potential discharge, a lower potential is provided whichcontinues the ionization at the trailing edge of the striation,producing a smearing of the striation, although ultimately thelowpotential discharge will be extinguished by the sweeping away of thegas. If high-potential pulses are provided at a rate determined interalia by the gas velocity and the gas conditions, a new high-potentialdischarge will occur just as the low-potential discharge which followedthe previous high-potential discharge ceases. Thus a continuum of highlyionized gas will be sustained along the flow axis; and by suitabledisposition of electrode pairs this may extend across substantially theentire cross section of the gas stream. Since the magnetically inducedfield is not of primary importance in the functioning of this method, itis useful to place the electrodes of a pair so that the axis joiningthem is parallel to the magnetic field existing (which in this case isnot essential but may be present from e.g. the main generator field) sothat the striations will not be distorted by interaction with it, in thefashion well known in the blow-out magnets used in circuit breakers. Adischarge parallel to a magnetic field, on the other hand, tends to bestabilized somewhat by the known focussing effect of a field sooriented.

The use of thermionic cathodes is desirable in order to reduce theelectrode drop and thus reduce the power required for ionization. Suchelectrodes may be cesiated, either of the dispenser type or, if thecesium present from seeding is sufficient, by the ambient cesium; orthey may be oxide-coated refractory cathodes, absent cesium. Variousways of providing adequate cesium to thermionic cathodes without raisingthe cesium concentration in the gas mass proper to undesirably highvalues are taught in my copending application, Ser. No. 178,881, filedSept. 9, 1971, entitled Thermionic Cathodes for MHD Generator,identified by docket number 39-SE-2021, and assigned to the assignee ofthis application, to which reference is made for its teachings.

The importance of my invention is essentially what may be described asphysically economic. The purpose of an MHD generator of the kinddescribed is to convert heart energy into electrical energy. For botheconomical and ecological reasons it is desirable that this be done asefficiently as possible. This requires that (a) the highest feasibletemperature be used, but this is limited by available materials, so (b)ionization means must be provided to render as efficient as possible agenerator operating at a permissibly low temperature. But ionizationrequires consumption of energy. The use of thermionically emissivecathodes reduces the energy requirements, as does the use of twodifferent potentials applied to the preionizing electrodes. Providingthe ionization means as close as possible to the generating channelminimizes the degree of ionization which such ionizing means mustproduce, since the ionization dies out with time, and the shorter thetime interval between ionization of a given volume of gas and its use ingeneration the less such loss will occur. Furthermore, when the thermalionization initially existing is sufficient for enhancement without theuse of energyconsuming means to produce initial ionization, the use of athermally insulated duct of large cross section so that the gas ispresented at approximately stagnation temperature, uncooled by largeadiabatic expansion, provides further economy. In an actual test of agenerator according to my invention it was found that the ionizationmeans consumed energy equal to only ten percent of the energy output ofthe generator; and the efficiency of the generation process itself wasenhanced by the high and uniform conductivity of the resulting ionizedgas, or plasma. Since the ionization renders the gas capable ofefficient use immediately upon its entry into the generator channel, thelength of generator channel required for a given output is alsominimized; since such channel is expensive to construct, such efficientutilization of the channel also reduces the cost of the installation,and its fixed charges.

BRIEF DESCRIPTION OF THE DRAWINGS:

FIG. 1 represents schematically and partly in section a source of hotseeded noble gas under pressure.

FIG. 2 represents schematically and partly in section a conventionalportion of an MHD generator and associated equipment.

FIG. 3 represents schematically and partly in section an embodiment ofmy invention.

FIG. 4 represents schematically and partly in section a source of hotionized noble gas under pressure.

FIGS. 5 and 6 represent schematically and partly in section plan andelevation of another embodiment of my invention.

FIG. 7 represents an electrical circuit which is suitable for use in myinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 represents aconventional source of heat 10 in thermal connection with a conventionalsource 12 of noble gas under pressure, heated by source 10, and seededwith cesium from seeder 14. Gas duct 16 is represented as furnishingreplenishment of noble gas to 12 from a source not shown in FIG. 1, andcesium duct 18 is represented as furnishing replenishment of cesium toseeder 14, from a source not shown in FIG. 1. Thus far, only theconventional art has been represented. Duct 20 is shown in section, andserves to carry seeded hot noble gas from source 12 at a velocity notgreater than 0.1 Mach, so that the gas is at substantially thestagnation conditions of pressure and temperature prevailing in source12. In order that cooling of duct 20 may not alter the temperaturesubstantially, it is provided with thermal insulation 22, also shown insection. Thus at the exit from duct 20, the gas will still be nearly ashot and hence as highly ionized as it is under stagnation conditions.

Delaying the disclosure of the various embodiments of preionizer inorder to complete the description of the more or less conventionalapparatus in which it functions, FIG. 2 represents the generator properwith certain conventional auxiliaries. Magnet poles 24 and 26 provide afield transverse to the flow axis of a refractory insulating (andtherefore ceramic) channel 28, shown in section. Its section normal tothe paper is rectangular, so that its top and bottom (which appear insection in FIG. 2) and its back wall, like the front wall which has beenremoved by the sectioning, are all planar. On the back wall of chamber28 there are represented a plurality of generator anodes 30, subdividedboth axially and also transversely to the channel axis (cf. US. Pat. No.3,525,000), electrically separate from each other, each having anelectrical connection extending through the channel back wall to theexterior of the channel 28 for connection to a load. In the front wallof the channel 28, which does not appear because it has been removed bythe sectioning, there are an equal number of generator cathodes,preferably thermionically emissive, which may profitably be inaccordance with the teachings of my copending application previouslyreferenced. For each power anode 30 there is a power cathode lyingopposed to it on a line orthogonal to the channel 28 axis (which ishorizontal in FIG. 2) and to the planes of the walls of channel 28 inwhich the electrodes are installed-that is, to the plane of FIG. 2. Thusthe generator electrodes form pairs of one generator anode and onegenerator cathode in each pair; and it is part of the known art that itis usually preferable to connect independent loads separately to eachpair of generator electrodes.

An exit duct 32 of refractory ceramic is shown, in section, as connectedto the end of channel 28 to lead the seeded noble gas to cooler-filter34, where the cesium vapor is condensed and the resulting droplets,together with any casual particles, are filtered out; and the noble gasis cooled and returned to compressor 36, which raises it to a pressuresufficient to permit its return, via duct 16, to source 12. The cesium,not liquid after its condensation, is similarly returned by pump 38 viaduct 18 to cesium seeder 14 for reuse. Except to the extent that theteachings of my referenced copending application are embodied in thethermionic cathodes, the embodiment represented in FIG. 2 isconventional in the known published art.

FIG. 3 represents in section a nozzle designated generally as 40, havingplanar bottom shown in FIG. 3, planar top not shown because removed bythe sectioning of the drawing, and side walls which appear in sectionand are singly curved-that is generated by the motion of a straight linemoved always parallel to its initial position but not in a single plane.Nozzle 40 has an entry portion 42 connected to duct 20, tapering to itsnarrowest part throat 44, and then having an expanding portion 46 whichis connected to the entry to channel 28, and may, indeed, be integralwith it in practice, so that the numerical references refer to regionsrather than necessarily physically separate parts. Indeed, four columnsof generator anodes 30 are represented, the projection in FIG. 3corresponding to plan view, from above, while that in FIG. 2 correspondsto an elevation from the side. Hence in FIG. 3 the generator anodes 30,which appear full face in vertical columns in FIG. 2, appear in profilein FIG. 3, and the top generator anode in each column masks those belowit. For simplicity in the drawing, the generator anodes 30 (and allother electrodes in FIG. 3) have been represented simply symbolically,with lines from them passing through the wall of nozzle 40 to representtheir electrical connection with the exterior of the nozzle. Inpractice, the anodes will conveniently be of a refractory metal such astungsten, and be of rather substantial cross section where they passthrough the wall in order that their temperature may be adjusted byappropriate thermal design. It is preferable that the anodes, includingthe generator anodes, be relatively cool, since they need not, andpreferably should not, emit electrons in significant density.

Generator cathodes 48 are represented in four columns in the wall ofexpanding portion 46 of nozzle 40 opposite the generator anodes 30. Thegenerator cathodes being equal in number to the generator anodes 30, andeach such cathode being opposed to and paired with one and onlygenerator anode. The generator cathodes 48 may be of refractory materialoxide coated; but they are preferably of tungsten cesiated with cesiumvapor in accordance with the teachings of my copending application.Their temperature at the faces exposed to the interior of nozzle 40should be appropriate to produce copious thermionic emission from thesurface. This temperature will depend upon the particular. emissivesurface provided, and it is desirable that the generator cathodes 48actually have a very substantial cross section extending through thenozzle wall in order that the temperature of the surface exposed to thehot gas may be controlled by external cooling or heating as theparticular design and operating conditions may dictate.

Upstream from the generator cathodes 48, extending back through throat44 and into entry portion 42, there are represented columns of ionizingcathodes 50, preferably identical with generator cathodes 48 anddiffering only in function. Similarly, opposed to ionizing cathodes 50there are represented columns of ionizing anodes 52, preferablyidentical with generator anodes 30. The ionizing cathodes 50 and anodes52 are paired in one to one relation precisely like the generator anodes30 and cathodes 48.

A circle with central dot, the conventional end-on representation of thehead of an arrow, 54 represents the direction of the magnetic fieldproduced by pole pieces 24 and 26 of FIG. 2. This will have its fullmagnitude in the generator channel 28, but its fringing will cause it toexist in reduced magnitude in nozzle 40, the magnitude in the nozzlebeing capable of adjustment by the exact design of the pole pieces 24and 26, which may be made wide enough to extend also over all or part ofnozzle 40.

In one mode of procedure, each ionizing cathode 50 is connectedelectrically to the ionizing anode 52 with which it forms a pair; eachsuch pair is insulated from all other such pairs. The hot seeded gas induct 20 is somewhat ionized, at approximately the stagnationtemperature. It is part of my teaching that nozzle 40 be madesufficiently short that the time of passage of a given volume of gasfrom duct 20 through the nozzle is less than the ionization relaxationtime of the seeded gas, so that the degree of ionization will at leastnot decrease markedly during such passage. However, the electricalconductivity produced by such ionization at permissible gas temperaturesis insufficient for efficient generator operation. Current flow betweenthe connected ionizing cathode and ionizing anode of each pair, producedby the Faraday field created by the motion of the gas ions through themagnetic field will enhance the initial ionization of the gas, so thatwhen it moves between the generator electrode pairs it will besufficiently conducting for efficient generation. In particular, theelectron average energy, or temperature, will be greatly increased.Since the gas velocity will in general be fixed by the desired operatingconditions, the magnetic field may be adjusted to provide currentadequate to provide an electron density of e.g. 5 X 10 per cubiccentimeter, which suffices for efficient generator operation. However,if the combination of current and magnetic field required to achievethis is so great that the resulting deceleration of the gas reachesobjectionable values, it may be more efficient to apply potentials froman external source to the electrodes of a pair. This may be applied onlyto the electrodes in the first few upstream columns of ionizingelectrodes, where the induced voltages will be low because the gasvelocity is low there; or it may be applied to all the ionizingelectrode pairs, in which event it is permissible to have the ionizingelectrodes so oriented that the line joining any pair is parallel to themagnetic field; or the magnetic field need not extend to the preionizingelectrode region. The method described in connection with FIG. 3 assumessome appreciable ionization in the gas entering the nozzle, of the orderof 10 per cubic centimeter, which will ordinarily be produced either bycesium seeding of a hot gas, but may be produced, if a radioactivesource of heat is employed, by ionization created in unseeded gas byirradiation from the source occuring simultaneously with the heating.FIG. 4 represents the modification of FIG. 1 with such a source wouldentail. Pressurized noble gas source 12 remains, as does duct 16 forreplenishing the supply of gas, but cesium seeder 14 is not present, andsimple heat source 10 is replaced by ionizing heat source 56. Sincethermal ionization is not particularly effective, the duct 58 leadingthe gas from source 12 need not be made of large cross section to avoidadiabatic cooling, but should be short since the gas ionization willbegin to decrease immediately upon the removal of the ionized gas fromthe vicinity of the ionizing heat source 56.

FIGS. and 6 represent an alternative manner of providing preionizationin a gas inadequately ionized (e.g. per cubic centimeter or less). Inthis embodiment, externally supplied electrical energy is relied uponcompletely to produce the requisite high uniform ionization, so that thenozzle length is of no importance, so long as it is consistent withefficient conversion of the pressure energy of the gas to kineticenergy. The ionizing electrodes are located in a channel which iseffectively part of the generator channel. FIG. 5 represents a sectionedtop view and FIG. 6 represents a sectioned elevation, so that thedescription will apply to both simultaneously. Refractory insulatingchannel 60 is provided at its upstream end, which is connected to theexit from the expansion nozzle to receive hot gas moving at highvelocity, with ionizing cathodes 62, similarly to the provision ofionizing cathodes 50 in FIG. 3. Opposed to each ionizing cathode 62 isan ionizing anode 64. The line joining an ionizing cathode 62 with itspaired ionizing anode 64 is parallel to the magnetic field representedby arrow 66. Generator anodes 68 are opposed to generator cathodes 70,paired one to one, the generator cathodes being thermionically emissivelike ionizing cathodes 62 and the other cathodes previously mentioned.Between each ionizing cathode 62 and its paired ionizing anode 64 thereis connected a source of electrical potential having the characteristicsof the embodiment represented by FIG. 7. A highvoltage pulse source 72is connected to pulse transformer 74, whose secondary is in parallel viadiode 75 with a low-voltage source 76 in series with a diode 78. Fromthis parallel combination leads extend to an ionizing anode 64 and anionizing cathode 62 of the same pair. The high-voltage pulse source 72produces a pulse of perhaps a microsecond duration which appears at thesecondary of pulse transformer 74, blocks diode 78,

and, being applied to ionizing anode 64 and ionizing cathode 62, breaksdown the gas between them, producing an ionization density of perhaps 10electrons per cubic centimeter. The discharge will be somewhat in theshape of a tube of ionized gas because of the focussing effect of themagnetic field 66. After the pulse has died out, diode 78 is unblockedand low-voltage source 76 sustains the ionization for a time; but thevolume of ionized gas is being swept down stream so that ultimately thelow-voltage discharge ceases. A new pulse from source 72 then initiatesa repetition of the cycle. The frequency of pulses from source 72 isadjusted to match the duration of the cycle; for a gas velocity of akilometer per second and a pulse voltage of a kilovolt, the repetitionrate or frequency of pulses from source will be of the order of 10kilohertz. Each pair of ionizing electrodes is connected to a separatecircuit having high-voltage pulse followed by longer low-voltage; butall high-voltage pulses are simultaneous. The spacing between successivecolumns of ionizing electrodes should be somewhat greater than thediameter of the discharge column produced by the high-voltage pulse. Thenet effect of the two-voltage discharge is that the initial pulseprovides an electron density of e.g. l0 per cubic centimeter, and theprolonged low-voltage discharge increases this to 10 or more; and theionization is substantially uniform. Experience has shown thathigh-voltage pulses alone produce separate columns of ionization whichyield very low generator efficiency.

If it is desired to increase the ionization above that produced by theembodiment described, the first few columns of electrode pairsrepresented as generator electrodes may be short-circuited and used likeionizing electrodes 42 and 50 of FIG. 3 to enhance the ionization stillfurther.

The variety of embodiments I have disclosed require a recapitulation ofthe common inventive concept they teach. There is a basic transitionpoint at about l0 electrons per cubic centimeter in the manner in whicha gas may be treated to further ionize it. If it has at least the statedfree electron concentration its ionization may be enhanced to thatrequired for MHD generator operation, i.e. 5 X 10 electrons per cubiccentimeter, by simple passage between pairs of short-circuited ionizingelectrodes in the presence of a transverse magnetic field; or the samepairs of ionizing electrodes may be employed, but with an externalpotential source rather than the Faraday field resulting from passage ofthe ionized gas transversely to the magnetic field. In either case,there is a voltage source in the circuit of each pair. In the generalcase including either mode of operation, the ionizing electrodes extendupstream along the flow axis to a source of gas having the requisiteelectron density of 10 electrons per cubic centimeter. Such gas may beprovided in numerous ways. It may be cesiated and heated, conductedthrough a thermally insulated duct of such cross section that thevelocity of the gas flow (and the required gas flow, obviously, will bewhatever is required for the generator to produce its required output)is not greater than 0.1 Mach. Or it may be ionized e.g. by radiationfrom a nuclear source which also provides the heat for operation of thegenerator, in which case the gas need not be conveyed at low velocity asin the preceding case, but should be conveyed to the ionizing electrodesrapidly in order that the ionization may not decrease appreciably beforereaching them. If the gas as provided from the source of hot noble gashas an electron density appreciably less than 10 electrons per cubiccentimeter, then the first few columns of what have been calledionization electrodes actually function as preionization electrodes tobeing the gas up to the requisite l electrons per cubic centimeter, andso serve-as the source of gas so ionized for the remaining ionizingelectrodes. It would be possible thus to employ preionizing electrodesas a source of gas so ionized for short-circuited pairs of ionizingelectrodes; but, while this might prove economical in some particulardesign, I prefer generally to apply to the ionizing electrodes the samecyclical potential, consisting of a first high-potential pulse followedby a longer low-potential pulse, which I apply to the electrode pairswhich serve as preionizing electrodes. It is necessary to define an axisfor the circuit formed through the gas between the ionizing anode andthe ionizing cathode of a given pair. Since the electrodes are of finitedimensions, the reference to a line joining them is theoreticallyinexact; but I use the term to mean a line joining their geometriccentroids. Similarly, when a magnetic field is described as normal ororthogonal to such a line, it is evident that my invention will notsuddenly cease to function if the magnetic field is at e.g. 89 or 91 tosuch a line, and that the term magnetic field refers in such case to theorthogonal or normal magnetic field component. Similarly, the use of theterm relaxation time with reference to the decay of ionization is notintended to be confined only to an exact mathematical period, but ratherto refer to a time such that the ionization decays substantially. Sinceall the physical laws involved (including those, such as ionization,which are statistical) are mathematically continuous, it is evident thatsmall deviations from recited conditions will produce only smalldeviations from the optimum, and may thus produce acceptable resultsdeparting only slightly from the optimum, and my claims are to beunderstood as including such small deviations which, in practice,necessarily occur.

What is claimed is: 1. In a magnetohydrodynamic generator comprising: a.a source of hot noble gas under pressure connected to b. a refractoryinsulating channel provided with c. a plurality of pairs of generatorelectrodes each consisting of a generator anode and a generator cathodelocated in opposition to each other in opposite walls of the channel,and having individual electrical connections extending through thechannel wall to the outside of the channel, and d. a generator magneticfield extending transverse to the axis of flow through the channel andtransverse to the line joining a generator anode and a generator cathodeof a same pair, the improvement comprising: ionization enhancement meanscomprising e. a plurality of pairs of ionization electrodes, each pairconsisting of a thermionically emissive cathode and an anode opposed tothe cathode, forming a path between the anode and cathode for flow ofionized gas orthogonally to a line between the anode and cathode;

f. connection means between the anode and the cathode, external to thegas flow path, forming a closed circuit for electron flow from thecathode through the gas to the anode, and back to the cathode throughthe connection means;

g. a voltage source in the said closed circuit to cause the flow ofcurrent in the said closed circuit;

h. the pairs of ionization electrodes being subdivided along the axis offlow of gas between them, and extending normally to the axis so that theportions of gas between them extend across substantially the entire gaspath,

and extending downstream along the axis to a point immediately adjacentto the generator electrodes and extending upstream along the axis to asource of gas having an electron density of at least 10" per cubiccentimeter.

2. The improvement claimed in claim I, further comprising:

i. an expansion nozzle having an entry portion and a throat and anexpanding portion,

j. the entry portion being connected to receive therein said hot noblegas under pressure having an electron density of at least 10 per cubiccentimeter;

k. the therein said pairs of ionization electrodes are located in thewalls of the entry portion and of the throat and of the expandingportion; and

l. the length of the expansion nozzle is such that the time of passageof the said gas through it is less than the ionization relaxation of thegas.

3. The improvement claimed in claim 2 in which m. the therein said hotnoble gas under pressure and having an electron density of at least 10per cubic centimeter is seeded with cesium; and

n. the source of the herein said gas is connected with the entry portionof the said expansion nozzle by a thermally insulated duct of crosssection sufficiently large that the velocity of gas flow through it isnot greater than 0.1 Mach.

4. The improvement claimed in claim 1 in which 0. the therein saidsource of hot noble gas under pressure provides gas having an electrondensity of less than l0 per cubic centimeter;

p. pairs of preionization electrodes identical with the pairs ofionization electrodes recited in h) of claim 1 are located upstream ofthe said pairs of ionization electrodes and q. there is applied to eachpair of preionization electrodes and to each pair of ionizationelectrodes a cyclical potential of which one cycle consists of a highpotential pulse followed by a lower potential pulse of longer durationthan the high potential pulse.

5. The improvement claimed in claim 1 in which the therein said voltagesource g) is provided by the interaction of the ionized gas movingbetween the ionizing cathode of an ionizing electrode pair and itsassociated ionizing cathode, with a magnetic field component normal tothe direction of motion of the gas and of a line joining the electrodesof a pair.

1. In a magnetohydrodynamic generator comprising: a. a source of hotnoble gas under pressure connected to b. a refractory insulating channelprovided with c. a plurality of pairs of generator electrodes eachconsisting of a generator anode and a generator cathode located inopposition to each other in opposite walls of the channel, and havingindividual electrical connections extending through the channel wall tothe outside of the channel, and d. a generator magnetic field extendingtransverse to the axis of flow through the channel and transverse to theline joining a generator anode and a generator cathode of a same pair,the improvement comprising: ionization enhancement means comprising e. aplurality of pairs of ionization electrodes, each pair consisting of athermionically emissive cathode and an anode opposed to the cathode,forming a path between the anode and cathode for flow of ionized gasorthogonally to a line between the anode and cathode; f. connectionmeans between the anode and the cathode, external to the gas flow path,forming a closed circuit for electron flow from the cathode through thegas to the anode, and back to the cathode through the connection means;g. a voltage source in the said closed circuit to cause the flow ofcurrent in the said closed circuit; h. the pairs of ionizationelectrodes being subdivided along the axis of flow of gas between them,and extending normally to the axis so that the portions of gas pathbetween them extend across substantially the entire gas path, andextending downstream along the axis to a point immediately adjacent tothe generator electrodes and extending upstream along the axis to asource of gas having an electron density of at least 1011 per cubiccentimeter.
 2. The improvement claimed in claim 1, further comprising:i. an expansion nozzle having an entry portion and a throat and anexpanding portion, j. the entry portion being connected to receivetherein said hot noble gas under pressure having an electron density ofat least 1011 per cubic centimeter; k. the therein said pairs ofionization electrodes are located in the walls of the entry portion andof the throat and of the expanding portion; and l. the length of theexpansion nozzle is such that the time of passage of the said gasthrough it is less than the ionization relaxation of the gas.
 3. Theimprovement claimed in claim 2 in which m. the therein said hot noblegas under pressure and having an electron density of at least 1011 percubic centimeter is seeded with cesium; and n. the source of the hereinsaid gas is connected with the entry portion of the said expansionnozzle by a thermally insulated duct of cross section sufficiently largethat the velocity of gas flow through it is not greater than 0.1 Mach.4. The improvement claimed in claim 1 in which o. the therein saidsource of hot noble gas under pressure provides gas having an electrondensity of less than 1011 per cubic centimeter; p. pairs ofpreionization electrodes identical with the pairs of ionizationelectrodes recited in h) of claim 1 are located upstream of the saidpairs of ionization electrodes and q. there is applied to each pair ofpreionization electrodes and to each pair of ionization electrodes acyclical potential of which one cycle consists of a high potential pulsefollowed by a lower potential pulse of longer duration than the highpotential pulse.
 5. The improvement claimed in claim 1 in which thetherein said voltage source g) is provided by the interaction of theionized gas moving between the ionizing cathode of an ionizing electrodepair and its associated ionizing cathode, wiTh a magnetic fieldcomponent normal to the direction of motion of the gas and of a linejoining the electrodes of a pair.